This invention relates generally to microwave circuits and more particularly to distributed microwave circuits.
As is known in the art, radio frequency distributed amplifiers including a plurality of successively interconnected field effect transistors are used to provide amplification of radio frequency signals. Such amplifiers are, in particularly, used in applications requiring broad frequency bandwidths of operation.
In a conventional distributed amplifier, an input propagation network successively couples input electrodes of a plurality of field effect transistors, for example, and an output propagation network successively couples output electrodes of the plurality of field effect transistors. Since field effect transistors have intrinsic input capacitance and output capacitance, in general, the presence of these capacitances limits the bandwidth of operation of the FET when used in an amplifier. However, with the distributed approach, the input and output capacitances of the FET become part of the propagation networks forming artificial transmission lines. In this manner, major band limiting effects of the input and output capacitances of the transistors in reducing frequency bands of operation of the amplifier are avoided.
Negative feedback is a commonly applied technique in low frequency amplifiers. Negative feedback is often used in a low frequency amplifier to provide, amongst other things, amplifiers having relatively low level distortion. There are many examples of negative feedback employed in low frequency amplifiers whereas at high frequencies, such as microwave frequencies, beyond about 1-2 gigahertz, for example, the examples of negative feedback are relatively few. In particular, those examples of negative feedback used at microwave frequency are generally confined over very narrow frequency bands of operation. One problem which has prevented use of negative feedback at microwave frequencies, particularly over broad frequency band of operation, is that the phase shift of a feedback path is related to the phase length of the signal path, as well as, the phase of the signal at the output of the amplifier. At microwave frequencies the electrical pathlength between the output and input of the amplifier may be appreciable, thus resulting in a variation in phase shift which is a function of frequency. Moreover, stability of such an amplifier is also a problem since positive feedback may also occur at some frequencies.
Further complicating the use of negative feedback at microwave frequencies is that approaches such as monolithic microwave integrated circuits provided as microwave amplifiers and other circuit elements use metal semiconductor field effect transistor (MESFET) or high electron mobility transistors (HEMTs) having gallium arsenide or other Group III-V material as an active layer as an active element of the circuit. Such devices are inverting devices. That is, the output signal is 180.degree. out of a phase with the input signal. Because of their parasitic capacitances, however, insertion phases vary significantly from 180.degree. at microwave frequencies. It is not uncommon, therefore, to have a field effect transistor which will have an insertion phase near 180.degree. below 1 GHz, but will drop down to 90.degree. at 10 GHz.